The overall objectives of this project are essentially the following (abbreviated statement): I. Analysis of malignancy by somatic cell fusion; II. Study of surface antigens and other markers by somatic cell fusion. Concomitant studies of viral cycle regulation, with regard to both DNA (EBV) and C-type (murine oncorna) viruses; III. Study of differentiation patterns in somatic hybrids; IV. In vivo fusion of tumor and host cells; V. Genetic analysis of tumor antigen recognition.